Methods and apparatus for intervertebral disc prosthesis insertion

ABSTRACT

A method for inserting an intervertebral disc prosthesis into a space between two vertebrae involves inserting the prosthesis partway into the space under constraint to prevent endplates of the prosthesis from articulating, releasing the prosthesis from constraint, and inserting the unconstrained prosthesis farther into the space. In some embodiments, the method involves grasping the prosthesis with a grasping device to insert the prosthesis partway under constraint, loosing the grasping device to release the prosthesis from constraint, and pushing the prosthesis farther into the disc space using the grasping device and/or one or more separate pusher devices. A system includes a grasping device, at least one separate pushing device, and optionally a vertebral spreading device and/or a vertebral midline indicator device.

CROSS REFERENCE TO RELATED APPLICATION DATA

The present application is a divisional of U.S. patent application Ser.No. 10/913,780 filed Aug. 6, 2004, the full disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to medical devices and methods. Morespecifically, the invention relates to intervertebral disc prostheses.

Back pain takes an enormous toll on the health and productivity ofpeople around the world. According to the American Academy of OrthopedicSurgeons, approximately 80 percent of Americans will experience backpain at some time in their life. In just the year 2000, approximately 26million visits were made to physicians' offices due to back problems inthe United States. On any one day, it is estimated that 5% of theworking population in America is disabled by back pain.

One common cause of back pain is injury, degeneration and/or dysfunctionof one or more intervertebral discs. Intervertebral discs are the softtissue structures located between each of the thirty-three vertebralbones that make up the vertebral (spinal) column. Essentially, the discsallow the vertebrae to move relative to one another. The vertebralcolumn and discs are vital anatomical structures, in that they form acentral axis that supports the head and torso, allow for movement of theback, and protect the spinal cord, which passes through the vertebrae inproximity to the discs.

Discs often become damaged due to wear and tear or acute injury. Forexample, discs may bulge (herniate), tear, rupture, degenerate or thelike. A bulging disc may press against the spinal cord or a nerveexiting the spinal cord, causing “radicular” pain (pain in one or moreextremities caused by impingement of a nerve root). Degeneration orother damage to a disc may cause a loss of “disc height,” meaning thatthe natural space between two vertebrae decreases. Decreased disc heightmay cause a disc to bulge, facet loads to increase, two vertebrae to rubtogether in an unnatural way and/or increased pressure on certain partsof the vertebrae and/or nerve roots, thus causing pain. In general,chronic and acute damage to intervertebral discs is a common source ofback related pain and loss of mobility.

When one or more damaged intervertebral discs cause a patient pain anddiscomfort, surgery is often required. Traditionally, surgicalprocedures for treating intervertebral discs have involved discectomy(partial or total removal of a disc), with or without fusion of the twovertebrae adjacent to the disc. Fusion of the two vertebrae is achievedby inserting bone graft material between the two vertebrae such that thetwo vertebrae and the graft material grow together. Oftentimes, pins,rods, screws, cages and/or the like are inserted between the vertebraeto act as support structures to hold the vertebrae and graft material inplace while they permanently fuse together. Although fusion often treatsthe back pain, it reduces the patient's ability to move, because theback cannot bend or twist at the fused area. In addition, fusionincreases stresses at adjacent levels of the spine, potentiallyaccelerating degeneration of these discs.

In an attempt to treat disc related pain without fusion, an alternativeapproach has been developed, in which a movable, implantable, artificialintervertebral disc (or “disc prosthesis”) is inserted between twovertebrae. A number of different intervertebral disc prostheses arecurrently being developed. For example, the inventors of the presentinvention have developed disc prostheses described in U.S. patentapplication Ser. Nos. 10/855,817 and 10/855,253, previously incorporatedby reference. Other examples of intervertebral disc prostheses are theLINK® SB CHARITTÉ™ disc prosthesis (provided by DePuy Spine, Inc.)MOBIDISK™ disc prosthesis (provided by LDR Medical), the BRYAN™ cervicaldisc prosthesis (provided by Medtronic Sofamor Danek, Inc.), thePRODISC™ disc prosthesis or PRODISC-C™ disc prosthesis (from SynthesStratec, Inc.), and the PCM™ disc prosthesis (provided by Cervitech,Inc.).

To insert an artificial intervertebral disc prosthesis, and indeed forperforming most disc-related surgeries, it is typically necessary togain access to the disc and the intervertebral space from an anterior toposterior direction (i.e., through the front of the patient), to avoidcoming in contact with the spinal cord. Thus, surgical procedures on adisc are typically approached anteriorly through the neck or abdomen,depending on which disc (or discs) is being repaired. Methods forinserting a disc prosthesis generally involve removing the damaged disc,preparing the surfaces of the two vertebral bones to receive theprosthesis, spreading the two vertebrae apart using one or morespreading devices, and inserting the prosthesis into the space betweenthe two vertebrae. Examples of such methods are described in U.S. Pat.Nos. 6,478,800, 6,235,030, 6,652,533, 6,689,132, 6,261,296 and6,666,866, and in U.S. Patent Application Nos. 2001/0031969,2001/0029377, 2003/0153916, 2002/0198532, 2004/0024407, 2003/0216737,2003/0204261, 2003/0135220 and 2003/0014114. Due to the invasive natureof such procedures, one important goal is to reduce invasiveness, thuscausing as little trauma to tissues surrounding the surgical site aspossible.

The main drawback of currently available methods for insertingintervertebral disc prostheses is that they require a larger amount ofspreading apart (or “distraction”) of the two vertebrae than is optimal.Over-distraction is necessary when using such methods because it isimportant to insert the disc prosthesis all the way into the disc space,to position the center of rotation of the prosthesis closer to theposterior portion of the vertebrae than to the anterior portion. Thisallows the vertebrae to move as they were intended and avoids placingundue strain on the facet joints of the vertebrae or on otherstructures. To push a prosthesis toward the back of a disc space,however, it is typically necessary to spread the two vertebrae apartwidely, since the anterior portion of the disc space is usually wider(or higher) than the posterior portion. In some methods, an artificialdisc is placed by placing a first endplate into the space, placing asecond endplate into the space, and then spreading the vertebrae wideenough to wedge a core in between the two endplates. In other methods,the vertebrae are spread apart as far as practicable, the wholeprosthesis is inserted while one or more spreading devices are in place,and not until the prosthesis is completely inserted is the spreadingdevice removed. In either case, as well as in other currently availablemethods, the two vertebrae adjacent the prosthesis are typically spreadfarther apart than would be desirable for a longer amount of time thanwould be desirable. In some cases, the posterior longitudinal ligament(PLL) is released, or “cut,” to enable sufficient distraction for discplacement.

Distracting vertebrae can damage muscles, ligaments, nerves and/or othertissues in and around the vertebral column. Such damage may actuallycause the patient to experience as much, or even more, pain aftersurgery than was caused by the original disc problem.

Therefore, as the use of intervertebral disc prostheses increases, anincreasing need exists for improved methods and apparatus for insertingsuch prostheses. Ideally, such intervertebral prosthesis insertionmethods and devices would provide for insertion of a prosthesis adesired distance into an intervertebral space while reducing the needfor intervertebral distraction, thus preventing or at least reducingtrauma to surrounding tissues. Also ideally, such insertion methods anddevices would be relatively simple and easy to use, thereby reducing theoverall invasiveness of the procedure. At least some of these objectiveswill be met by the present invention.

2. Description of the Background Art

A number of exemplary intervertebral disc prostheses are listed above.Published US patent applications 2002/0035400A1 and 2002/0128715A1describe disc implants which comprise opposing plates with a corebetween them over which the plates can slide. The core receives one ormore central posts, which are carried by the plates and which locate inopposite ends of a central opening in the core. Such arrangements limitthe load bearing area available between the plates and core.

Other patents related to intervertebral disc prostheses include U.S.Pat. Nos. 4,759,766; 4,863,477; 4,997,432; 5,035,716; 5,071,437;5,370,697; 5,401,269; 5,507,816; 5,534,030; 5,556,431; 5,674,296;5,676,702; 5,702,450; 5,824,094; 5,865,846; 5,989,291; 6,001,130;6,022,376; 6,039,763; 6,139,579; 6,156,067; 6,162,252; 6,315,797;6,348,071; 6,368,350; 6,416,551; 6,592,624; 6,607,558 and 6,706,068.Other patent applications related to intervertebral disc prosthesesinclude U.S. Patent Application Publication Nos. 2003/0009224;2003/0074076; 2003/0191536; 2003/0208271; 2003/0135277; 2003/0199982;2001/0016773 and 2003/0100951. Other related patents include WO01/01893A1, EP 1344507, EP 1344506, EP 1250898, EP 1306064, EP 1344508,EP 1344493, EP 1417940, EP 1142544, and EP 0333990.

BRIEF SUMMARY OF THE INVENTION

The present invention generally provides methods for inserting anintervertebral disc prosthesis, as well as devices and systems forperforming the methods. One advantage of these improved methods is thata disc prosthesis may be inserted with minimal or reduced intervertebraldistraction, thus avoiding trauma to tissues in and around the insertionsite. Reduced distraction is generally achieved by inserting aprosthesis into an intervertebral space while allowing endplates of theprosthesis to articulate during at least part of the insertion process.For the purposes of this application, to “articulate” means to moverelative to another structure. Thus, allowing endplates of anintervertebral prosthesis to articulate means that endplates are free tomove relative to each other, relative to a core of the prosthesis,relative to a ball and socket joint of the prosthesis, relative to amobile or fixed center of rotation of the prosthesis and/or the like.Although the following description often focuses on disc prostheseshaving two endplates and a core, various embodiments of the insertionmethod may be applied to any other intervertebral disc prosthesis.Articulation of the endplates during insertion allows the prosthesis tobe pushed posteriorly into a disc space without excessive intervertebraldistraction or significant forces being applied to the vertebrae, thusachieving desirable positioning of the prosthesis while avoiding traumato surrounding muscles, ligaments, nerves and the like.

In one aspect of the present invention, a method of inserting anintervertebral prosthesis into a space between two adjacent vertebraeinvolves inserting the prosthesis partway into the space underconstraint to prevent endplates of the prosthesis from articulating,releasing the prosthesis from constraint, and inserting theunconstrained prosthesis farther into the space. As mentioned above, insome embodiments, the endplates of the constrained prosthesis areprevented from articulating about a core of the prosthesis, while theendplates of the unconstrained prosthesis are generally free toarticulate about the core to help the prosthesis conform to the spacebetween the two vertebrae. In alternative embodiments, the unconstrainedendplates may be free to articulate about a ball and socket joint orother structure.

In one embodiment, inserting the prosthesis partway under constraintinvolves grasping the endplates with an insertion tool such that theycannot move relative to the core and pushing the prosthesis partway intothe space using the insertion tool. In such embodiments, releasing theprosthesis from constraint may involve loosening the insertion tool. Insome embodiments, the loosened insertion tool may then be used to insertthe unconstrained prosthesis farther into the intervertebral space.Additionally, or alternatively, a separate pusher tool may be used toinsert the unconstrained prosthesis farther into the space.

In some embodiments, inserting the constrained prosthesis partway intothe space between the vertebrae involves inserting the prosthesis lessthan halfway into the space. In one embodiment, for example, theconstrained prosthesis is inserted about one third of the way into thespace and then subsequently inserted farther into the space. Inalternative embodiments, the constrained prosthesis may be pushed morethan halfway into the space, less than one third of the way into thespace, or any other suitable distance into the space. The unconstrainedprosthesis is then inserted any desired distance farther into theintervertebral space. In some embodiments, for example, the prosthesisis inserted sufficiently far into the space that a center of rotation ofthe prosthesis is closer to the posterior edges of the vertebrae than tothe anterior edges of the vertebrae.

In some embodiments, the prosthesis is inserted in approximately ananterior to posterior direction. In alternative embodiments, theprosthesis may be inserted in an anterolateral-to-posterior direction,lateral-to-lateral direction or posterior-to-anterior direction.Typically, inserting the unconstrained prosthesis farther into the spaceinvolves pushing the prosthesis into the space. In other embodiments,however, techniques other than pushing may be used to insert theprosthesis, such as pulling. In some embodiments, pushing the prosthesisfarther into the space involves individually pushing upper and lowerendplates of the prosthesis. Alternatively, or additionally, the upperand lower endplates of the prosthesis may be simultaneously pushed intothe intervertebral space. In various embodiments, individual andsimultaneous endplate pushing may be performed using the graspingdevice, a separate pusher device, or both.

In some embodiments, the method also includes inserting a vertebralspacing device at least partway into the space and manipulating thespacing device to increase a height of the space. An example of such aspacing device is described by the assignees of the present applicationin PCT Patent Application Number 2004/000171, filed Jan. 26, 2004, thefull disclosure of which is hereby incorporated by reference. Thespacing step is typically performed before inserting the constrainedprosthesis partway into the intervertebral space. In some embodiments,inserting the constrained prosthesis partway into the intervertebralspace comprises sliding the prosthesis between two opposing jaws of thespacing device. In such an embodiment, inserting the constrainedprosthesis partway into the space may optionally further involve slidingat least one fin on at least one outer surface of the endplates throughat least one corresponding slot in the opposing jaws. In one embodiment,such a fin (or fins) may then be slid into a corresponding slot formedin one of the vertebrae.

Some embodiments also involve using a vertebral midpoint indicatordevice to locate a midpoint of at least one of the two vertebrae, andmarking the midpoint on one or both of the two vertebrae. An example ofa midline indicator device is described by the assignees of the presentapplication in PCT Patent Application Number 2004/000170, filed Jan. 26,2004, the full disclosure of which is hereby incorporated by reference.Midline finding and marking are typically performed before inserting theconstrained prosthesis partway into the intervertebral space. In someembodiments, locating the midpoint involves inserting the vertebralmidpoint indicator device into the space between the vertebrae andimaging the midpoint indicator device using a radiographic imagingdevice.

In another aspect of the present invention, a method of inserting anintervertebral prosthesis into a space between two adjacent vertebraeinvolves sliding the prosthesis partway into the space between thevertebrae between two opposing jaws of a spacing device, removing thespacing device from the space to release the prosthesis from constraint,and pushing the unconstrained prosthesis farther into the space whileallowing endplates of the prosthesis to articulate. In this method, theendplates of the prosthesis are constrained from articulating when theprosthesis is disposed between the jaws. Again, in some embodiments, theunconstrained endplates articulate about a core of the prosthesis, whilein alternative embodiments they may articulate about a ball and socketjoint or other structure. Such a method may optionally further includeinserting the spacing device at least partway into the space andmanipulating the spacing device to increase a height of the space. Anyof the additional or alternative features described above may also beapplied in various embodiments.

In another aspect of the present invention, a device for inserting anintervertebral prosthesis into a space between two adjacent vertebraeincludes: an elongate rigid shaft having a proximal end and a distalend; an adjustable grasping member coupled with the distal end forreleasably grasping endplates of the intervertebral prosthesis; and anactuator disposed near the proximal end of the shaft and coupled withthe grasping member for adjusting the grasping member to grasp andrelease the prosthesis. The grasping member is adapted to grasp theprosthesis such that the endplates are constrained from articulating andsuch that the outer diameter of the grasping member does not extendbeyond a largest diameter of the endplates. Additionally, the graspingmember is adapted for pushing the prosthesis into the space between thetwo vertebrae either while grasping the prosthesis or after releasingthe prosthesis.

In some embodiments, the grasping member comprises movable opposing jawsadapted to simultaneously grasp inner rims of the endplates. Such agrasping member may, for example, be coupled with the actuator via atleast one rod extending through the shaft. For example, the graspingmember may include a first half coupled with a first movable rodextending from the actuator and a second half coupled with a secondmovable rod extending from the actuator. In some embodiments, theactuator comprises a thumb screw adapted to move the first and secondrods closer together and farther apart by turning the thumb screw inopposite directions. In alternative embodiments, the actuator mayinclude, but is not limited to, a trigger, tongs and a movable handle.In a number of embodiments, the device is shaped to pass between twoopposable jaws of a vertebral spacing device disposed between the twoadjacent vertebrae. Also in some embodiments, the grasping member may beadapted to push the endplates either simultaneously or individually intothe space.

In another aspect of the present invention, a system for inserting anintervertebral prosthesis into a space between two adjacent vertebraeincludes a prosthesis grasping device for grasping the prosthesis andpushing the prosthesis at least partway into the space and at least oneprosthesis pusher device for pushing the prosthesis farther into thespace. The grasping device is similar to the device describedimmediately above and may include any of the described features. Thepusher device may include: an elongate shaft having a proximal end and adistal end; a concave pusher portion disposed at the distal end, thepusher portion adapted to push the endplates either simultaneously orindividually into the space; and a handle disposed at the proximal end.

In some embodiments, the system further includes a vertebral spacingdevice for increasing a height of the space between the two vertebrae.An example of such a spacing device is described in PCT PatentApplication Number 2004/000171, which was previously incorporated byreference. In such embodiments, the grasping member is adapted to slidethrough the vertebral spacing device disposed between two adjacentvertebrae. Optionally, the pusher device may also be adapted to slidethrough the spacing device.

The system may optionally further include a vertebral body midlineindicator device for locating a midline on a vertebral body of at leastone of the two vertebrae. An example of a midline indicator device isdescribed in PCT Patent Application Number 2004/000170, which waspreviously incorporated by reference.

These and other aspects and embodiments will be described in furtherdetail below, with reference to the drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E demonstrate a method for inserting an intervertebral discprosthesis according to one embodiment of the present invention.

FIGS. 2A and 2B are top views of a grasping device for inserting anintervertebral disc prosthesis according to one embodiment of thepresent invention.

FIG. 2C is a side view of the distal end of the device in FIGS. 2A and2B.

FIG. 3 is a top view of a pusher device for inserting an intervertebraldisc prosthesis according to one embodiment of the present invention.

FIGS. 4A-4E are various views of a spreader device for distracting twoadjacent vertebrae for inserting an intervertebral disc prosthesisaccording to one embodiment of the present invention.

FIG. 5 demonstrates the spreading action of the spreader device in FIGS.4A-4E.

FIGS. 6A-6D are various views of a vertebral body midline indicatordevice according to one embodiment of the present invention.

FIGS. 7A and 7B demonstrate indication of vertebral midline using thedevice in FIGS. 6A-6D.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1A-1E, in one embodiment a method for inserting anintervertebral disc prosthesis 104 into an intervertebral space ISbetween two adjacent vertebrae V first involves inserting the discprosthesis 104 partway into the space IS while the prosthesis 104 isconstrained (FIG. 1A). By “constrained” it is meant that endplates 106of the prosthesis 104 are not free to articulate (move) about a core 112(FIGS. 1B-1E) of the prosthesis 104. To insert the prosthesis 104partway under constraint, an insertion device 102 may be used. Such aninsertion device 102 may suitably include a grasping member 110 coupledwith an elongate shaft 108. At an end opposite the grasping member 110(not shown), the insertion device 102 may include a handle, an actuatorto control the grasping member 110 and/or any other suitable features,some of which are described further below.

The prosthesis 104 may be inserted as far into the intervertebral spaceIS under constraint as is desired. In some embodiments, for example, theprosthesis 104 is inserted under constraint approximately one-third ofthe way into the space IS. In other embodiments, the prosthesis 104 maybe inserted less than one-third of the way, closer to one-half of theway, or any other suitable distance into the space IS.

As shown in FIG. 1B, once the prosthesis 104 is inserted partway underconstraint, the insertion device 102 may be removed, thus releasing theprosthesis 104 from constraint. From this point forward, the endplates106 of the prosthesis 104 are free to move about the prosthesis core112. Examples of such a prosthesis 104 with endplates 106 and core 112are described by the assignees of the present application in U.S. patentapplication Ser. Nos. 10/855,817 and 10/855,253, previously incorporatedby reference, although any other suitable prosthesis may be used invarious embodiments.

Referring now to FIGS. 1C-1E, in some embodiments the insertion device102 may be used to push the unconstrained prosthesis 104 farther intothe intervertebral space. In some embodiments, one or more separatepusher devices (not shown) may be used in addition to or instead of theinsertion device 102 for pushing the prosthesis 104 farther into thespace IS. FIGS. 1C and 1D demonstrate that in one embodiment thegrasping member 110 of the insertion device 102 is adapted to pushindividually against the upper (FIG. 1C) and lower (FIG. 1D) endplates106. As shown in FIG. 1E, the grasping member 110 may also be adapted topush simultaneously against the upper and lower endplates 106, thuspushing the prosthesis 104 as a unit farther into the intervertebralspace IS.

By inserting the prosthesis 104 farther into the space IS while it isunconstrained, thus allowing the endplates 106 to articulate about thecore 112, the method reduces the need for increasing the height of theintervertebral space IS by distracting the vertebrae V away from eachother. Because the endplates 106 are free to articulate, the prosthesis104 is better able to conform to the intervertebral space IS, thusreducing trauma to the vertebrae V and also limiting trauma tosurrounding structures caused by over-distraction.

The unconstrained prosthesis 104 may be inserted as far into theintervertebral space IS as is desired. In some embodiments, for example,the prosthesis 104 is pushed far enough into the space IS so that acenter of rotation of the prosthesis 104 is closer to a posterior edge P(FIG. 1E) of the vertebrae V than to an anterior edge A of the vertebraeV. In alternative embodiments, any other suitable insertion distance ordepth may be used. Once a desired amount of insertion is achieved, theinsertion device 102 is removed and the prosthesis 104 is in placebetween the two adjacent vertebrae V.

In various embodiments, the method just described may include fewersteps or additional steps. For example, in one embodiment, a spreaderdevice is inserted between the two vertebrae V to spread them apartbefore inserting the constrained prosthesis 104. An example of such aspacing device is described in PCT Patent Application Number2004/000171, previously incorporated by reference. In such embodiments,the insertion device 102 is typically sized to fit between opposing jawsof the spreader device. When the prosthesis 104 is partially inserted,the spreader device is removed from the intervertebral space IS, and theprosthesis 104 is released from constraint and inserted the rest of theway into the space IS. Also in some embodiments, a midline indicatordevice may be used to facilitate the location of a midline on one orboth of the two adjacent vertebrae V. An example of such a midlineindicator device is described in PCT Patent Application Number2004/000170, previously incorporated by reference. Typically, themidline indicator is used before the disc prosthesis 104 is inserted.These and other steps or features may be included in various embodimentsof the method without departing from the scope of the invention.

Referring now to FIGS. 2A-2C, one embodiment of an insertion device 120for inserting an intervertebral disc prosthesis 140 suitably includes anelongate shaft 126, a grasping member 122 coupled with the distal end ofthe shaft 126, and a handle 128 at the proximal end of the shaft 120,including one or more actuators 130 for controlling movement of thegrasping member 122. One or more rods 124 or other connectors extendfrom the grasping member 122 through the shaft 126 to the actuator 130.In the embodiment shown, for example, the grasping member 122 comprisestwo opposable tongs or jaws, which may be moved closer together orfarther apart (double-headed arrows) via the actuator 130 and rods 124.The actuator 130 shown is a thumb screw. In alternative embodiments,scissor-type mechanisms, spring loaded tongs, a triggering mechanism orany other suitable grasping and actuating means may be used. Anysuitable material or combination of materials may be used to manufacturethe insertion device, including but not limited to stainless steeland/or other suitable metals.

As shown in FIG. 2B, the insertion device 120 may grasp a discprosthesis 140 such that the grasping member 122 does not protrudebeyond an outer edge 141 of the prosthesis 140. In other words, thegrasping member 122 holds onto an inner portion of the prosthesis 140,so that it will not extend beyond the lateral edges 141 of theprosthesis 140. This configuration is advantageous during insertion, asthe grasping member 122 is essentially out of the way, within the outeredge 141 of the prosthesis 140.

FIG. 2C is a side view of a distal end of the insertion device 120 shownin FIGS. 2A and 2B. It is more readily seen that the disc prosthesis 140includes a core 146 and two endplates 142. Each endplate 142 includes aninner rim 144 that contacts the core 146 and a fin 148 for enhancingattachment to vertebral bone. The grasping member 122 of the insertiondevice 120 grasps the inner rims 144 of the endplates 142, thuspositioning it within the outer edges 141 of the endplates 142. Ofcourse, in various embodiments of the methods described herein, anysuitable alternative prosthesis may be used, as well as any suitableinsertion device (or devices).

Referring now to FIG. 3, in some embodiments a separate pusher device150 may be used to push an unconstrained prosthesis 140 farther into anintervertebral space. The pusher device 150 is typically constructed ofstainless steel or other suitable metal and suitably includes anelongate shaft 152, a pusher member 154 at the distal end of the shaft152, and a handle 158 at the proximal end of the shaft 152. The pushermember 154 includes a concave inner portion 156 for pushing against theinner rims 144 of endplates 142 of the prosthesis 140. The concaveportion 156 may be tapered and/or rounded to facilitate pushing againstupper and lower endplates 142 individually while also allowing forsimultaneous pushing against both endplates 142. In alternativeembodiments, the pusher device 150 may have any of a number ofalternative configurations, shapes, sizes and the like. In someembodiments, multiple pusher devices 150 of different configurationsand/or sizes are provided to allow a physician to select one or moredesired devices 150.

Referring now to FIGS. 4A-4E, a spreader device 10 for spreadingadjacent vertebrae to facilitate intervertebral disc prosthesis 30insertion is shown. Again, the device 10 is described in greater detailin PCT Patent Application Number 2004/000171, which was previouslyincorporated by reference. The spreader device 10 generally includesdistally located opposable jaws 12, a slidable pusher member 45 and anactuator 15. The opposable jaws 12 are carried by arms 14 which formpart of a scissors-type mechanism having a single hinge point 15.Handles 16 on the proximal end of the device are used to manipulate theopposable jaws 12. When the handles 16 are actuated, arms 14 translatethe actuation motion to the single hinge point scissors type mechanism15. This causes the opposable jaws 12 to open or close. The jaws 12 haveopposing surfaces 18 formed with ribs 20 and transverse slots 22 whichextend for the height of the jaws as seen in FIG. 4B. At their freeends, the jaws 12 are provided with relatively sharp tips or blades 24having curved extremities 26.

FIGS. 4A and 4B illustrate how the handles 14 are inclined relative tothe jaws 12. Manipulation of the handles 16 by moving them causes thejaws 12 to open or close. Other embodiments include a double hingeinstead of the single hinge 15 which would pivot the jaws apart from oneanother when the handles 16 are displaced towards one another.

The insertion device 10 illustrated in FIGS. 4A-4E is designed forplacement of an intervertebral prosthetic disc 30. Such a prostheticdisc 30 comprises opposing endplates 32 which are located on oppositesides of a central core 34. The opposing endplates 32 articulate aboutthe central core 34. The prosthetic disc 30 also comprises projectingfins 36 which are aligned with matching slots 40 in the vertebrae 38during implantation. Typically slots 40 are saw cut into the vertebrae38.

A method of inserting the intervertebral prosthesis is illustrated inFIGS. 4D and 5. In order to place the prosthesis 30, the vertebrae 38are distracted by a distance sufficient for at least partial insertionof the prosthesis 30. To achieve this, the tips 24 of the opposable jaws12 are inserted between the vertebrae 38 with the slots 22 in theopposable jaws 12 aligned with the slots in the vertebrae 40. Thehandles 16 are then manipulated to force the opposable jaws 12 apartwhich also forces the vertebrae 38 apart from one another, creating agap. The prosthesis 30 is then inserted into the gap 42 between theopposable jaws 12 where it is held therein with fins 36 engaged with thecorresponding slots 22. The prosthesis 30 is then slipped distally inthe gap while being guided by the fins 36 cooperating with the slots 22.The prosthesis 30 is moved through the inter-jaw gap and past the jawtips 24 in order to locate the prosthesis 30 between the vertebrae 38with fins 36 in the vertebral cut slots 40. The slots 22 in theopposable jaws 12 help to guide the fins 36 into the vertebral cut slots40.

FIG. 4C illustrates the jaws 12 inclined towards one another, in thedirection towards the tips 24. The gap 42 between the jaws 12 at the topis large enough for insertion of the prosthesis 30 between them at thatpoint. Therefore, in an alternative method of placing the prosthesis,the prosthesis 30 may be located initially in the gap 42 and then it maybe pushed down towards the tip 24, forcing the jaws 12 open andsimilarly forcing the vertebrae 38 apart from one another. A pusher 45may be used to hold, position and drive the prosthesis 30 during theplacement procedure. A force may be applied manually to pusher 45 or itmay be tapped on the upper end to drive the prosthesis downward.

Alternatively, the prosthesis placement procedure may be modified sothat the initial distraction of the vertebra 38 is achieved bymanipulation of the handles 16 and then a force may be applied manuallyto the pusher 45 or it may be tapped in order to create the finalintervertebral gap and placement of the prosthesis 30. The spreaderdevice 10 serves both to facilitate insertion of the prosthesis 30between the vertebrae 38 and also to ensure that the prosthesis 30 isaccurately guided into position with its fins 36 lined up with thevertebral slots 40.

FIG. 5 shows in greater detail (solid-tipped arrows) the various motionsinvolved in inserting the spreader device 10 into the intervertebralspace and manipulating the handles 16 to force open the jaws 12 and thusincreased the height of the intervertebral space between the twoadjacent vertebrae 38. As mentioned above, use of this or other spreaderdevices 10 is optional and is not required in all embodiments.

FIGS. 6A-6D show another optional device for use in the insertionmethods of the present invention. As mentioned above, a midlineindicator device 210 such as the one shown is described in greaterdetail in PCT Patent Application Number 2004/000170, which waspreviously incorporated by reference. The midline indicator 210 suitablyincludes an elongate shaft 212 and a body 214 coupled with one end ofthe shaft 212. The shaft 212 may be made of one or more radiopaquematerials, such as but not limited to stainless steel, titanium or thelike. Alternatively, the shaft 212 may be radiolucent. The body 214 ismade of one or more radiolucent materials, such as a polymer, so that itis not visible on radiographs. Embedded in the body 214 are two elongateradiopaque markers 216, also made of any suitable radiopaquematerial(s). The markers 216 are parallel to the shaft 212 and arelocated on opposite sides and equidistant from the shaft 212.

FIGS. 7A and 7B demonstrate a method for using the midline indicator tofind a vertebral body midline 222. FIG. 7A shows, in anterior view,adjacent upper 218 and lower 220 vertebrae. To determine the midline222, the surgeon uses the shaft 212 to insert the body 214 between thevertebrae 218, 220. The surgeon then attempts to position the shaft 212at the vertebral midline 222, and a radiograph is taken of the vertebrae218, 220 and indicator 210 from the anterior-posterior (A-P) direction.The surgeon then examines the radiograph to determine whether themarkers 216 are equidistant laterally from the lateral osseous edges 223of the vertebrae 218, 220—i.e., that the distance 225 is the same onboth sides, and that the markers 216 are aligned with the pedicles.

Additionally, if the shaft 212 and markers 216 are properly aligned inthe A-P direction, they will appear as dots on the radiograph. If themidline indicator 210 is turned, however, as is demonstrated by thedotted lines in FIG. 7B, the shaft 212 and markers 216 will show up aslines or stretched-out dots on the radiograph. The A-P direction of theradiograph is shown by 224, with misalignment of the indicator 210 shownby angles θ. By consulting one or more radiographs and manipulating theindicator 210, the surgeon positions the handle 212 of the indicator 210at the vertebral midline 222. The surgeon may then make a mark 226 inone or more vertebrae 218, 220 to indicate the midline 222. The mark 226may be made by any suitable means, such as by burning with anelectrocautery device, marking with a marking pen, inserting a pin, orthe like. After one or more midline marks 226 are made, the midlineindicator 210 is removed and the disc prosthesis (not shown) isinserted. Again, the midline finding step is optional.

Although the foregoing is a complete and accurate description of theinvention, any suitable modifications, additions or the like may be madeto the various embodiments without departing from the scope of theinvention. Therefore, nothing described above should be interpreted aslimiting the scope of the invention as it is described in the followingclaims.

1. A method of inserting an intervertebral prosthesis into a spacebetween two adjacent vertebrae, the method comprising: sliding theprosthesis partway into the space between the vertebrae between twoopposing jaws of a spacing device, wherein endplates of the prosthesisare constrained from articulating when disposed between the jaws;removing the spacing device from the space to release the prosthesisfrom constraint while partway into the space; and pushing theunconstrained prosthesis from partway into the space farther into thespace while allowing endplates of the prosthesis to articulate.
 2. Amethod as in claim 1, wherein the endplates of the constrainedprosthesis are prevented from articulating about a core of theprosthesis, and wherein the endplates of the unconstrained prosthesisare free to articulate about the core to help the prosthesis conform tothe space between the two vertebrae.
 3. A method as in claim 1, furthercomprising: inserting the spacing device at least partway into thespace; and manipulating the spacing device to increase a height of thespace.
 4. A method as in claim 3, wherein sliding the prosthesis partwayinto the space comprises sliding at least one fin on at least one outersurface of the prosthesis through at least one corresponding slot in theopposing jaws.
 5. A method as in claim 4, wherein pushing the prosthesisfarther into the space comprises sliding the at least one fin into atleast one corresponding slot formed in at least one of the twovertebrae.
 6. A method as in claim 1, wherein sliding the prosthesispartway into the space comprises inserting the prosthesis less thanhalfway into the space before removing the spacing device.
 7. A methodas in claim 1, wherein sliding the prosthesis partway into the spacecomprises: grasping the endplates with an insertion tool such that theycannot move relative to the core; and pushing the prosthesis between thejaws using the insertion tool.
 8. A method as in claim 7, furthercomprising releasing the prosthesis from the insertion tool beforeremoving the spacing device.
 9. A method as in claim 8, wherein pushingthe prosthesis farther into the space is performed using the insertiontool.
 10. A method as in claim 8, wherein pushing the prosthesis fartherinto the space is performed using a separate pusher tool.
 11. A methodas in claim 1, wherein the prosthesis is inserted in approximately ananterior-to-posterior direction.
 12. A method as in claim 1, wherein theprosthesis is inserted in approximately a lateral-to-lateral direction.13. A method as in claim 1, wherein the prosthesis is inserted in ananterolateral-to-posterior direction.
 14. A method as in claim 1,wherein pushing the unconstrained prosthesis farther into the spacecomprises positioning a center of rotation of the prosthesis closer toposterior edges of the two vertebrae than to anterior edges of thevertebrae.
 15. A method as in claim 1, wherein pushing the unconstrainedprosthesis farther into the space comprises individually pushing upperand lower endplates of the prosthesis.
 16. A method as in claim 1 or 15,wherein pushing the unconstrained prosthesis farther into the spacecomprises simultaneously pushing upper and lower endplates of theprosthesis.
 17. A method as in claim 1, further comprising, beforesliding the prosthesis partway into the space: locating a midpoint of atleast one of the two vertebrae using a vertebral midpoint indicatordevice; and marking the midpoint on one of the two vertebrae.
 18. Amethod as in claim 17, wherein locating the midpoint comprises:inserting the vertebral midpoint indicator device into the space betweenthe vertebrae; and imaging the midpoint indicator device using aradiographic imaging device.